Issue |
A&A
Volume 693, January 2025
|
|
---|---|---|
Article Number | A285 | |
Number of page(s) | 22 | |
Section | Planets, planetary systems, and small bodies | |
DOI | https://doi.org/10.1051/0004-6361/202451680 | |
Published online | 24 January 2025 |
Connection between planetary He I λ10 830 Å absorption and extreme-ultraviolet emission of planet-host stars
1
Centro de Astrobiología, CSIC-INTA,
Camino bajo del Castillo s/n,
28692
Villanueva de la Cañada, Madrid,
Spain
2
Instituto de Astrofísica de Andalucía (IAA-CSIC),
Glorieta de la Astronomía s/n,
18008
Granada,
Spain
3
Thüringer Landessternwarte Tautenburg,
Sternwarte 5,
07778
Tautenburg,
Germany
4
Institut für Astrophysik und Geophysik, George-August-Universität,
Friedrich-Hund-Platz 1,
37077
Göttingen,
Germany
5
Instituto de Astrofísica de Canarias (IAC),
38205
La Laguna,
Tenerife,
Spain
6
Departamento de Astrofísica, Universidad de La Laguna (ULL),
38206
La Laguna,
Tenerife,
Spain
7
Landessternwarte, Zentrum für Astronomie der Universität Heidelberg,
Königstuhl 12,
69117
Heidelberg,
Germany
8
Institut de Ciències de l’Espai (ICE, CSIC),
Campus UAB, Can Magrans s/n,
08193
Bellaterra,
Barcelona,
Spain
9
Institut d’Estudis Espacials de Catalunya (IEEC),
08860
Castelldefels,
Barcelona,
Spain
★ Corresponding author; jsanz@cab.inta-csic.es
Received:
27
July
2024
Accepted:
12
December
2024
Context. The detection of the He I λ10 830 Å triplet in exoplanet atmospheres has opened a new window for probing planetary properties, including atmospheric escape. Unlike Lyman α, the triplet is significantly less affected by interstellar medium (ISM) absorption. Sufficient X-ray and extreme ultraviolet (XUV) stellar irradiation may trigger the formation of the He I triplet via photoionization and posterior recombination processes in the planet atmospheres. Only a weak trend between stellar XUV emission and the planetary He I strength has been observed so far.
Aims. We aim to confirm this mechanism for producing near-infrared He I absorption in exoplanetary atmospheres by examining a substantial sample of planetary systems.
Methods. We obtained homogeneous measurements of the planetary He I line equivalent width and consistently computed the stellar XUV ionizing irradiation. Our first step was to derive new coronal models for the planet-host stars. We used updated data from the X-exoplanets database, archival X-ray spectra of M-type stars (including AU Mic and Proxima Centauri), and new XMM-Newton X-ray data recently obtained for the CARMENES project. These data were complemented at longer wavelengths with publicly available HST, FUSE, and EUVE spectra. A total of 75 stars are carefully analyzed to obtain a new calibration between X-ray and extreme ultraviolet (EUV) emission.
Results. Two distinct relationships between stellar X-ray emission (5–100 Å) and EUVH (100–920 Å) or EUVHe (100–504 Å) radiation are obtained to scale the emission from late-type (F to M) stellar coronae. A total of 48 systems with reported planetary He I λ 10 830 Å studies, including 21 positive detections and 27 upper limits, exhibit a robust relationship between the strength of the planetary He I feature and the ionizing XUVHe received by the planet, corrected by stellar and planetary radii, as well as the planet’s gravitational potential. Some outliers could be explained by a different atmospheric composition or the lack of planetary gaseous atmospheres. This relation may serve as a guide to predict the detectability of the He I λ 10 830 Å absorption in exoplanet atmospheres.
Key words: planets and satellites: atmospheres / planets and satellites: general / stars: coronae / ultraviolet: planetary systems / X-rays: stars
© The Authors 2025
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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